Spin stabilized artillery projectiles are gyroscopically stabilized, spinning rapidly about the projectile's longitudinal axis resulting from the action of the rifling during the launch sequence. In free flight after muzzle exit, aerodynamic forces act on the projectile body, producing a complex epicyclic motion of nutation and precession throughout the trajectory that may affect, and otherwise interfere with, a desired trajectory of the projectile.
As the range capability of artillery weapons and ammunition grows, accuracy and precision of delivery become increasingly important. Total delivery errors for standard, unguided 155 mm artillery projectiles, including all error sources, can exceed 300 meters at 30 km, while a point target size may be less than ten square meters. In such a case, the probability of hitting a specific point target at extended range will be low unless a large number of rounds are fired. A number of schemes have been proposed to provide some measure of control over the flight path of spin-stabilized projectiles, all aimed at enhancing the accuracy and precision of artillery fire sufficiently to improve the chance of impact at point targets at extended ranges with reduced expenditure of ammunition and without inflicting collateral damage on objects located in the vicinity of the desired target.
Previously proposed methods of trajectory correction fall into one of several generic types. There are known device, commonly called “dragsters,” that act to abruptly increase the drag of the projectile at some point in the flight of the projectile, causing the projectile to fall towards the target. There are also devices that have wings, known as “canards,” that are attached to a forward portion of the projectile. Some designs have fixed wings or canards, while others initially package the canards within the projectile, deploying only when trajectory adjustment is desired. There are also thruster schemes proposed that employ explosive charges or small thruster rocket motors to apply lateral force to the projectile during flight.
The previously proposed methods of trajectory correction are generally operationally limited or require complex implementation that may not be cost effective, such that none of the above-described methods have been adapted into widespread use. For example, dragster devices must be fired to over-shoot the target, and can only correct for down-range errors, not cross-range errors. Thus, dragster devices are often termed one dimensional correctors. Meteorological data that is not up-to-date (“stale MET”), or that is gathered at a location some distance from the projectile, may result in substantial cross-range errors that may not be corrected by one-dimensional dragster devices.
Canard devices may substantially increase drag of the projectile when deployed, thereby decreasing efficiency. Canards and their actuating mechanisms may also occupy large volumes of restricted space within the projectile, and require substantial power resources to operate. The relatively high drag of canard devices when deployed to control the projectile flight path may restrict the use of canard devices, in practice, to the terminal phase of the trajectory to avoid unacceptable range penalties. However, deployment late in the trajectory may reduce the total correction capability (“maneuver authority”) of the canard devices. Moreover, it may not be practical to arrange the canards to be retractable as well as deployable because of power, weight and complexity constraints.
Thruster devices may need to be small to fit within the restricted available space of the projectile, and the trajectory correction capability of the thruster devices may be strictly limited. For thrusters positioned other than near the center of mass, thruster operation may induce excessive oscillations that affect accuracy in projectile angle of attack.
Accordingly, it would be beneficial to provide a system for spin stabilized projectile trajectory control that is simple, effective and cost efficient to implement and operate.